busybox: let buildroot handle stripping

For some ABI's (like FLAT), we cannot strip busybox as the file format
does not support it and strip will abort with errors like 'File format
not recognized'.  Since the toplevel already takes care of stripping
things, simply disable the busybox step.

Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>